Data transmission system for a downhole component

ABSTRACT

The invention is a system for transmitting data through a string of downhole components. In accordance with one aspect of the invention, the system includes a plurality of downhole components, such as sections of pipe in a drill string. Each component has a first and second end, with a first communication element located at the first end and a second communication element located at the second end. Each communication element includes a first contact and a second contact. The system also includes a coaxial cable running between the first and second communication elements, the coaxial cable having a conductive tube and a conductive core within it. The system also includes a first and second connector for connecting the first and second communication elements respectively to the coaxial cable. Each connector includes a conductive sleeve, lying concentrically within the conductive tube, which fits around and makes electrical contact with the conductive core. The conductive sleeve is electrically isolated from the conductive tube. The conductive sleeve of the first connector is in electrical contact with the first contact of the first communication element, the conductive sleeve of the second connector is in electrical contact with the first contact of the second communication element, and the conductive tube is in electrical contact with both the second contact of the first communication element and the second contact of the second communication element.

RELATED APPLICATIONS

The present patent document claims the benefit of the filing date under35 U.S.C. §119(e) of Provisional U.S. Patent Application Ser. No.60/444,100, filed Jan. 31, 2003, which is hereby incorporated in itsentirety by reference.

GOVERNMENT INTEREST

This invention was made with government support under Contract No.DE-FC26-01NT41229 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND

The present invention relates to the field of electrical connectors,particularly connectors for coaxial cables. The preferred connectors areparticularly well suited for use in harsh environments wherein it isdesirable to seal the connection from the elements. One such applicationis in data transmission systems for downhole environments, such as alonga drill string used in oil and gas exploration or along the casings andother equipment used in oil and gas production.

The goal of accessing data from a drill string has been expressed formore than half a century. As exploration and drilling technology hasimproved, this goal has become more important in the industry forsuccessful oil, gas, and geothermal well exploration and production. Forexample, to take advantage of the several advances in the design ofvarious tools and techniques for oil and gas exploration, it would bebeneficial to have real time data such as temperature, pressure,inclination, salinity, etc. Several attempts have been made to devise asuccessful system for accessing such drill string data. One such systemis disclosed in co-pending U.S. application Ser. No. 09/909,469 (alsopublished as PCT Application WO 02/06716) which is assigned to the sameassignee as the present invention. The disclosure of this U.S.application Ser. No. 09/909,469 is incorporated herein by reference.

SUMMARY

Briefly stated, the invention is a system for transmitting data througha string of downhole components.

In accordance with one aspect of the invention, the system includes aplurality of downhole components, such as sections of pipe in a drillstring. Each component has a first and second end, with a firstcommunication element located at the first end and a secondcommunication element located at the second end. Each communicationelement includes a first contact and a second contact. The system alsoincludes a coaxial cable running between the first and secondcommunication elements, the coaxial cable having a conductive tube and aconductive core within it. The system also includes a first and secondconnector for connecting the first and second communication elementsrespectively to the coaxial cable. Each connector includes a conductivesleeve, lying concentrically within the conductive tube, which fitsaround and makes electrical contact with the conductive core. Theconductive sleeve is electrically isolated from the conductive tube. Theconductive sleeve of the first connector is in electrical contact withthe first contact of the first communication element, the conductivesleeve of the second connector is in electrical contact with the firstcontact of the second communication element, and the conductive tube isin electrical contact with both the second contact of the firstcommunication element and the second contact of the second communicationelement.

The first and second communication elements are preferably inductivecoils, and the inductive coils are preferably formed by a single loop ofwire. More preferably, the inductive coils include at least one loop ofwire set in circular trough of a magnetically conducting, electricallyinsulating material, preferably ferrite. Preferably, the trough isformed of segments of a magnetically conducting electrically insulatingmaterial, with the electrically insulating material segments preferablyretained within a groove formed in a metal ring.

In accordance with another aspect of the invention, the components aresections of drill pipe, each having a central bore, and the first andsecond communication elements are located in a first and second recessrespectively at each end of the drill pipe. The system further includesa first passage passing between the first recess and the central boreand a second passage passing between the second recess and the centralbore. The first and second connectors are located in the first andsecond passages respectively. Preferably, each section of drill pipe hasa portion with an increased wall thickness at both the box end and thepin end with a resultant smaller diameter of the central bore at the boxend and pin end, and the first and second passages run through theportions with an increased wall thickness and generally parallel to thelongitudinal axis of the drill pipe.

In accordance with another aspect of the invention, the system includesa first and second expansion plug, each of which includes a centralpassage and each of which is press-fit within the conductive tube so asto maintain the increased outside diameter of the conductive tube withinthe larger diameter portions of the first and second passagesrespectively. The system also preferably includes a first and secondretaining plug, each of which includes ridges on its outer surface toretain the expansion plugs in place.

In accordance with another aspect of the invention, the first and secondcommunication elements each includes an inductive coil having at leastone loop of wire. In each communication element, there is a water-tightseal between the wire and the inside of the conductive tube. Thewater-tight seal preferably includes at least one gasket through whichthe first end of the wire passes and which forms a seal with the innersurface of the conductive tube.

The invention also includes a method of electrically connectingcommunication elements at opposite ends of a downhole component througha coaxial conductor. The method includes providing a coaxial cable asthe conductor between the first and second communication elements. Thecoaxial cable includes a conductive tube, a conductive core within theconductive tube and a dielectric material between the conductive tubeand the conductive tube. The method also includes providing a first andsecond connector for connecting the first and second respectivecommunication elements to the coaxial cable. The first and secondconnectors each include a conductive sleeve that fits around and makeselectrical contact with the conductive core. The conductive sleeve iselectrically isolated from the conductive tube. The method also includesremoving a portion of the dielectric material at both ends of thecoaxial cable to provide clearance for the conductive sleeve, andsliding the first and second connectors over both ends of the coaxialcable.

In accordance with another aspect of the invention, the method includesexpanding the outside diameter of the conductive tube by inserting anexpansion plug into each end. The first and second communicationelements each include an inductive coil having at least one loop ofwire. In each communication element, a first end of the wire is inelectrical contact with the conductive tube and a second end of the wireis in electrical contact with the conductive sleeve. The method furtherincludes inserting a water-tight seal between the second end of the wireand the inside of the conductive tube.

The present invention, together with attendant objects and advantages,will be best understood with reference to the detailed description belowin connection with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a section of drill pipe with cutawaysections showing the data transmission system.

FIG. 2 is a cross-sectional view along line 2—2 of FIG. 1.

FIG. 3 is a cross sectional view along line 3—3 of FIG. 1.

FIG. 4 is a cross-sectional view showing the pin end of FIG. 2 connectedto the box end of FIG. 3.

FIG. 5A is an enlarged cross-section of a connection betweencommunication elements of a connected pin and box end.

FIG. 5B is an enlarged cross-section of a connection betweencommunication elements of a connected pin and box end, showing theprotective bridge on the pin end.

FIG. 5C is an enlarged cross-section of a connection betweencommunication elements of a connected pin and box end, showing theprotective bridge on the box end.

FIG. 6 is a cross-sectional view of the pin end of a drill pipe showingthe connector.

FIG. 7 is an enlarged cross-sectional view from FIG. 6 showing theplacement of the magnetically connecting, electrically insulating (MCEI)element in the recess of the pin end of a drill pipe.

FIG. 8 is an enlarged cross-sectional view from FIG. 6 showing theplacement of the expansion plug, retaining plug, and water-tight seal.

FIG. 9 is an enlarged cross-sectional view from FIG. 6 showing theplacement of the centering guide.

FIG. 10 is an enlarged cross-sectional view from FIG. 6 showing theconnector and the end of the coaxial cable.

FIG. 11 is a perspective view of the communication element and steelring.

FIG. 11A is a perspective view showing a cross section of thecommunication element.

FIG. 12 is perspective view of the wire and the wire protection bridge.

FIG. 13 is an enlarged perspective view showing the water-tight seal.

FIG. 14 is an enlarged perspective view of FIG. 12 showing the wireprotection bridge.

FIG. 15 is a perspective view of the conductive tube and connectionelements.

FIG. 16 is an enlarged perspective view of FIG. 15 showing the centeringguide and the connector.

FIG. 17 is an enlarged perspective view of FIG. 15 showing the expansionplug and the retaining plug.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

It should be noted that, as used herein, the term “downhole” is intendedto have a relatively broad meaning, including such environments asdrilling in oil and gas, gas and geothermal exploration, the systems ofcasings and other equipment used in oil, gas and geothermal production.

It should also be noted that the term “transmission” as used inconnection with the phrase data transmission or the like, is intended tohave a relatively broad meaning, referring to the passage of signals inat least one direction from one point to another.

It should further be noted that the term “magnetically conductive”refers to a material having a magnetic permeability greater than that ofair.

It should further be noted that the term “electrically insulating” meanshaving a high electrical resistivity, preferably greater than that ofsteel.

Referring to the drawings, FIG. 1 is a perspective view of a section ofdrill pipe with cutaway sections showing the data transmission system ofthe present invention. The most preferred application of the connectoris in the data transmission system in sections of drill pipe, which makeup a drill string used in oil and gas or geothermal exploration.

The depicted section 15 of FIG. 1 includes a pin end 13, having externaltapered threads 19 (see FIG. 2), and a box end 11, having internaltapered threads 21 (see FIG. 3). Between the pin end 13 and box end 11is the body of the section. A typical length of the body is between 30and 90 feet. Drill strings in oil and gas production can extend as longas 20,000 feet, which means that as many as 700 sections of drill pipeand downhole tools can be used in the drill string.

There are several designs for the pin and box end of drill pipe. Atpresent, the most preferred design to use with the present invention isthat which is described in U.S. Pat. No. 5,908,212 to Grant Prideco,Inc. of Woodlands, Tex., the entire disclosure of which is incorporatedherein by reference. As shown in FIG. 2, the pin end 13 includes anexternal, primary shoulder 37, and an internal, secondary shoulder orface 35. As shown in FIG. 3, the box end 11 includes an external,primary shoulder 38 and an internal, secondary shoulder or face 36. Asshown in FIG. 4, when two sections of drill pipe are connected, the pinend 13 is threaded into the box end 11 with sufficient force so that theprimary external shoulder 37 on the pin end engages the primary shoulderface 38 on the box end. As a result of this connection being indexed bythe primary shoulder 37 and the primary shoulder face 38, the face 35 onthe pin end is reliably brought into close proximity or contact with theshoulder 36 on the box end. The advantages this provides to the presentinvention will be discussed below.

As shown in FIG. 2, the pin end 13 preferably includes a recess 32 inthe secondary or internal shoulder or face 35. Preferably, the recess islocated so as to lie equidistant between the inner and outer diameter ofthe secondary shoulder or face 35. Alternatively, the recess is formedat either the inner or the outer diameter of the face, thereby creatinga recess that is open on two sides.

Preferably, the recess is machined into the face by conventional toolseither before or after the tool joint is attached to the pipe. Thedimensions of the recess can be varied depending on various factors. Forone thing, it is desirable to form the recess in a location and with asize that will not interfere with the mechanical strength of the pinend. Further, in this orientation, the recesses are located so as to besubstantially aligned as the joint is made up. Other factors will bediscussed below.

As can be seen in these figures, the recess is preferably configured soas to open axially, that is, in a direction parallel to the length ofthe drill string. However, in alternative embodiments, the recesses maybe configured so as to open radially, that is, in a directionperpendicular to the length of the drill string. This offsetconfiguration does not materially affect the performance of theinductive elements of the present invention whether in an axial orradial configuration.

Referring to FIGS. 1, 2, and 3, lying within the recesses 32 and 34formed in the internal pin face 35 and internal shoulder face 36respectively is a communication element. As will be discussed below, thepreferred communication element is an inductive coil. However, othercommunication elements, such as acoustic transceivers, optic fibercouplers and electrical contacts are also benefited by being placed in arecess formed in the internal pin face and internal shoulder face. Inparticular, placing the communication elements in recesses withininternal faces provides for better protection from the harsh drillingenvironment. Also, when using a pipe joint such as that shown in FIG. 4that also includes external abutting faces 37 and 38, the internal faces35 and 36 are brought together in a more reliable manner. That is, withthe primary load taken by the external faces 37 and 38, the internalfaces 35 and 36 are brought together with a more consistent force.Preferably, the internal faces are less than about 0.03″ apart when theadjacent components are fully threaded together. More preferably, theinternal faces are touching. Most preferably, the internal faces are ina state of compression.

Returning to a discussion of the preferred embodiment with inductivecoils as the communication elements, it is noted that a typical drillpipe alloy, 4140 alloy steel, having a Rockwell C hardness of 30 to 35,has a magnetic permeability of about 42. The magnetic permeability of amaterial is defined as the ratio of the magnetic flux density Bestablished within a material divided by the magnetic field strength Hof the magnetizing field. It is usually expressed as a dimensionlessquantity relative to that of air (or a vacuum). It is preferable toclose the magnetic path that couples the adjacent coils with a materialhaving a magnetic permeability higher than the steel. However, if themagnetic material is itself electrically conducting, then it provides analternate electrical path to that offered by the adjacent loops. Thecurrents thus generated are referred to as eddy currents; these arebelieved to be the primary source of the losses experienced in prior-arttransformer schemes. Since the magnetic field is in a direction curlingaround the conductors, there is no need for magnetic continuity in thedirection of the loop.

In the preferred embodiment illustrated in FIGS. 2, 3 and 11, there islocated within the recess 32 a communication element 90 and within therecess 34 an identical communication element 92. In the preferredembodiment, the communication element consists of a steel ring 45containing a magnetically conducting, electrically insulating (MCEI)element 89, and a conductive coil 87 lying within the MCEI.

One property of the MCEI element is that it is magnetically conducting.One measure of this property is referred to as the magnetic permeabilitydiscussed above. In general, the magnetically conducting componentshould have a magnetic permeability greater than air. Materials havingtoo high of a magnetic permeability tend to have hysteresis lossesassociated with reversal of the magnetic domains themselves.Accordingly, a material is desired having a permeability sufficientlyhigh to keep the field out of the steel and yet sufficiently low tominimize losses due to magnetic hysteresis. Preferably, the magneticpermeability of the MCEI element should be greater than that of steel,which is typically about 40 times that of air, more preferably greaterthan about 100 times that of air. Preferably, the magnetic permeabilityis less than about 2,000. More preferably, the MCEI element has amagnetic permeability less than about 800. Most preferably, the MCEIelement has a magnetic permeability of about 125.

In order to avoid or reduce the eddy currents discussed above, the MCEIis preferably electrically insulating as well as magneticallyconductive. Preferably, the MCEI element has an electrical resistivitygreater than that of steel, which is typically about 12 micro-ohm cm.Most preferably, the MCEI has an electrical resistivity greater thanabout one million ohm-cm.

The MCEI element 89 is preferably made from a single material, whichitself has the properties of being magnetically conductive andelectrically insulating. A particularly preferred material is ferrite.Ferrite is described in the on-line edition of the EncyclopediaBritannica as “a ceramic-like material with magnetic properties that areuseful in many types of electronic devices. Ferrites are hard, brittle,iron-containing, and generally gray or black and arepolycrystalline—i.e., made up of a large number of small crystals. Theyare composed of iron oxide and one or more other metals in chemicalcombination.” The article on ferrite goes on to say that a “ferrite isformed by the reaction of ferric oxide (iron oxide or rust) with any ofa number of other metals, including magnesium, aluminum, barium,manganese, copper, nickel, cobalt, or even iron itself.” Finally, thearticle states that the “most important properties of ferrites includehigh magnetic permeability and high electrical resistance.”Consequently, some form of ferrite is ideal for the MCEI element in thepresent invention. Most preferably, the ferrite is one commerciallyavailable from Fair-Rite Products Corp., Wallkill, N.Y., grade 61,having a magnetic permeability of about 125. Another preferredcommercial supplier of ferrite is Gascyl Ent., Coquitlan, B.C., Canada.There are a number of other manufacturers that provide commercialproducts having a corresponding grade and permeability albeit underdifferent designations.

As an alternative to using a single material that is both magneticallyconductive and electrically insulating, the MCEI element can be madefrom a combination of materials selected and configured to give theseproperties to the element as a whole. For example, the element can bemade from a matrix of particles of one material that is magneticallyconductive and particles of another material that is electricallyinsulating, wherein the matrix is designed so as to prevent theconduction of electrical currents, while promoting the conduction of amagnetic current. One such material, composed of ferromagnetic metalparticles molded in a polymer matrix, is known in the art as “powderediron.” Also, instead of a matrix, the MCEI element may be formed fromlaminations of a material such as a silicon transformer steel separatedby an electrically insulating material, such as a ceramic, mineral(mica), or a polymer. Because the induced electric field is alwaysperpendicular to the magnetic field, the chief requirement for the MCEIelement is that the magnetic field be accommodated in a direction thatwraps around the coil, whereas electrical conduction should be blockedin the circumferential direction, perpendicular to the magnetic fieldand parallel to the coil.

In a more preferred embodiment shown in FIGS. 11 and 11A, thecommunication element 91 contains an MCEI element. The MCEI element isformed from several segments of ferrite 83 which are held together inthe appropriate configuration by means of a resilient material, such asan epoxy, a natural rubber, polytetrafluoroethylene (PTFE),perfluoroalkoxy (PFA), a fiberglass or carbon fiber composite, or apolyurethane. A preferred method of forming a segmented MCEI elementbegins with providing a steel ring 45 having a generally u-shaped troughconforming to the final dimensions of the MCEI element. In a preferredembodiment, the steel ring 45 has ridges 99 around its circumference inorder to enhance the connection of the steel ring to the drill pipe.

The element 99 is preferably manufactured as a complete unit and is theninserted into the drill pipe, the final assembly configuration beingshown in FIGS. 8, 9, and 10. To manufacture the element 99, a two-part,heat-curable epoxy formulation is mixed in a centrifuge cup. If theferrite elements have some porosity, they can be sealed by beingcentrifuged for up to 30 minutes to cause all bubbles induced by mixingto rise out of the viscous liquid, and to cause the liquid to penetrateand seal any porosity in the ferrite. Most preferably, a grade offerrite is used which has very low porosity which does not requiresealing in this fashion. The individual u-shaped ferrite segments arethen placed in the metal ring, except for a gap surrounding theretaining bridge 43, as shown in FIGS. 12 and 14.) Any excess epoxy iswiped out of the u-shaped groove. The upper surfaces of the parts can beprecisely aligned with each other by holding them in position withmagnets placed around the u-shaped trough in the mold. The epoxy is thencured, either at room temperature or in an oven.

The entire communication element 91, including the retaining bridge 43and the wire 41, may be preassembled before the communication element 91is inserted in the drill pipe, which can optionally be done in thefield. The steel ring 45 has the advantage that it provides a durableframe upon which to house the relatively fragile MCEI. The communicationelement 91 may be retained in the recess 32 of FIG. 2 by means of apolymeric bonding material, preferably epoxy, polyurethane,polytetrafluoroethylene, or perfluoroalkoxy, most preferably epoxy. Mostpreferably, the communication element 91 is retained in recess 32 bymeans of a press fit.

As seen in FIGS. 11 and 11A, the communication element 91 preferablycomprises a steel ring 45, an MCEI element, and a conductive coil 87.Lying within the trough 83 of the MCEI element 89 is the electricallyconductive coil 87. This coil is preferably made from at least one loopof an insulated wire, most preferably only a single loop. The wire ispreferably made of copper, most preferably of silver-plated copper-cladsteel, and is and insulated with varnish, enamel, or a polymer. Mostpreferably, the wire is insulated with a tough, flexible polymer such ashigh density polyethylene or polymerized tetrafluoroethane (PTFE). Thediameter of the wire, with insulation, is preferably selected so as tobe slightly less than the width of the U-shaped trough 83 in the MCEIelement. As will be discussed below, the specific properties of the wireand the number of loops is important in providing proper impedance forthe coil 87.

As shown in FIGS. 11 and 14, the communication element 91 has a firstand second contact for connecting to the coaxial cable 51. The firstcontact is preferably one end of the coil 87. The first contact ispreferably retained in the communication element by a retention bridge43. The retention bridge 43 is preferably inserted in a hole in thesteel ring 45, holding the first contact in place and preventing thefirst contact from coming into electrical contact with the secondcontact. The retention bridge 43 is made from an electrically insulatingmaterial, preferably PTFE, more preferably PEEK®. PEEK® is a trademarkfor a linear aromatic, semi-crystalline, polyetheretherketonethermoplastic polymer manufactured by Victrex PLC. A typical supplierfor such material is Zeus Products, Orangeburg, S.C. The second contactof the communication element 91 is in electrical contact with the steelring 45, preferably by means of a welded connection 85.

For a given application, the transformer diameter is fixed by thediameter of the pipe. The impedance of the transformer, and its desiredoperating frequency, can be adjusted by two factors: the number of turnsin the conductor and the ratio of length to area of the magnetic path,which curls around the conductors. Increasing the number of turnsdecreases the operating frequency and increases the impedance.Lengthening the magnetic path, or making it narrower, also decreases theoperating frequency and increases the impedance. This is accomplished byincreasing the depth of the U-shaped trough or by decreasing thethickness of the side-walls. Adjusting the number of turns gives a largeincrement, while adjusting the dimensions of the trough enables smallincrements. Accordingly, the invention allows the impedance of thetransformer portion of the transmission line to be precisely matched tothat of the conductor portion, which is typically in the range of 30 to120 ohms. Although an insulated copper wire is preferred, otherelectrically conductive materials, such as silver or copper-coatedsteel, can be used to form the coil 87.

As can be seen in FIG. 11, in a preferred embodiment the coil 87 isembedded within a material which fills the space within the trough ofthe MCEI element 89. Naturally, this material should be electricallyinsulating. It is also preferable that this material is resilient so asto add further toughness to the MCEI element. The preferred material touse for this purpose is a two-part epoxy formulation, preferably onefilled with a powdered material such as fumed silica or fine aluminumoxide to provide abrasion resistance. The applicants have used standardcommercial grade epoxy combined with a ceramic filler material, such asaluminum oxide, in proportions of about 50/50 percent. Other proportionsmay be desirable, but the filler material should not be less than 3percent nor greater than 90 percent in order to achieve suitableabrasion resistance as well as adequate adhesiveness. Alternatively,other materials, such as room-temperature curable urethanes, are used.It is important that the material be able to withstand the extremeconditions found downhole. Consequently, it is important to treat thematerial in such a way as to ensure the absence of voids or air pockets.

As can be seen in FIG. 3, the box end 11 also includes a recess 34similar to the recess 32 in the pin end, except that the recess 34 isformed in the internal, secondary shoulder 36 of the box end. Acommunication element 92, similar in all respects to the communicationelement 90, is located within the recess 34.

As can be seen in FIGS. 4, 5A, 5B, and 5C, when the pin and box end arejoined, the communication element 90 of the pin end and thecommunication element 92 of the box end are brought to at least closeproximity. Preferably, the elements 90 and 92 are within about 0.5 mm ofeach other, more preferably within about 0.25 mm of each other. Mostpreferably, the elements 90 and 92 are in contact with each other. FIG.5A is an enlarged cross-section of a connection between communicationelements 90, 92 of a connected pin and box end. FIG. 5B is an enlaroedcross-section of a connection between communication elements 90, 92 of aconnected pin and box end, showing the protective bridge 43 on the pinend. FIG. 5C is an enlarued cross-section of a connection betweencommunication elements 90, 92 of a connected pin and box end, showingthe protective bridge 43 on the box end.

Because the faces 35 and 36 of the pin and box end may need to bemachined in the field after extended use, it may preferred to design thetroughs in the pin and box end with a shape and size so as to allow thefirst and second conductive coils to lie in the bottom of the respectivetroughs and still be separated a distance from the top of the respectivefirst and second sides. As a result, the faces 35 and 36 can be machinedwithout damaging the coils lying at the bottom of the troughs. In thisembodiment, this distance is preferably at least about 0.01 inches, morepreferably, this distance is at least about 0.06 inches.

Turning to FIGS. 2 and 3, the passages 23 and 25 are holes, preferablydrilled from one point in the bottom of the recess 32 and 34,respectively, through the enlarged wall of the pin end and box end,respectively, so that the holes open into the central bore of the pipesection 15. The diameter of the hole will be determined by the thicknessavailable in the particular joint. For reasons of structural integrityit is preferably less than about one half of the wall thickness.Preferably, these holes have a diameter of about between 3 and 7 mm. Ascan be seen from FIGS. 2, 3, and 8, the diameter of the passages 23 and25 preferably increases slightly towards the pin recess 32 and the boxrecess 34. These larger diameter sections towards the pin recess 32 andthe box recess 34 are called the pin connector channel 31 and the boxconnector channel 33.

These two holes can be drilled by conventional means. Preferably, theyare drilled by a technique known as gun drilling. Preferably, therecesses can be machined and the holes can be drilled in the field, soas to allow for retrofitting of existing drill pipe sections with thedata transmission system of the present invention in the field.

A conductive tube 71 is placed within the passages 23 and 25.Preferably, the conductive tube 71 runs almost the entire length of thedrill pipe, beginning in the pin end connector channel 31, continuingthrough the pin end passage 23, passing through the hole 93 to enter theinterior of the body of the pipe section, entering hole 95, continuingthrough the box end passage 25, and ending near the box end connectorchannel 33. The conductive tube 71 is preferably held in tension afterit is inserted in the drill pipe 15 and remains in tension duringdownhole use. This prevents the conductive tube 71 from moving relativeto the passages 23 and 25 during downhole use. The conductive tube ispreferably made of metal, more preferably a strong metal, mostpreferably steel. By “strong metal” it is meant that the metal isrelatively resistant to deformation in its normal use state. The metalis preferably stainless steel, most preferably 316 or 316L stainlesssteel. A preferred supplier of stainless steel is Plymouth Tube,Salisbury, Md.

The elements of a preferred embodiment of the invention, from thecommunication element to the coaxial cable, are shown in FIGS. 6 through10. FIGS. 7 through 10 are enlarged cross sectional views of FIG. 6 fromright to left, with FIG. 7 showing an enlarged view of the right end ofFIG. 6, FIGS. 8 and 9 showing enlarged views of the center, and FIG. 10showing an enlarged view of the left side of FIG. 6.

In a preferred embodiment of the invention, the conductive tube is heldin place in each end by means of an expansion plug 61 and a retainingplug 63, as shown in FIGS. 6 and 8. The expansion plug 61 preferablyincreases in diameter from front 62 to back 64, such that the diameterof the back 64 is larger than the initial inner diameter of theconductive tube 71. The expansion plug 61 has a center opening throughwhich the wire 41 passes and is preferably made of metal, morepreferably tool steel, most preferably Viscount 44 steel. Thus, as theexpansion plug 61 is inserted in the conductive tube 71, the diameter ofthe conductive tube 71 is increased. The expansion plug 61 is insertedup to a distance relatively near the transition point 97, where thediameter of the passage 31 or 33 undergoes a change in diameter. Theresult of this insertion of the expansion plug 61 is that the diameterof the conductive tube 71 is larger on each end, so that the conductivetube 71 is held in place in the passages 31 and 33.

In a preferred embodiment, the expansion plug 61 is held in place by aretaining plug 63, as shown in FIG. 8. The retaining plug 63 is placedin the conductive tube 71 after the expansion plug 61 and has a centeropening through which the wire 41 passes. The retaining plug 63 is mademetal, more preferably tool steel, most preferably Viscount 44 steel. Ina preferred embodiment, the retaining plug 61 has ridges along its outerdiameter to dig into the inner diameter of the conductive tube 71 andhold the expansion plug 61 in place.

After exiting the holes 93 and 95, the conductive tube 71 passes throughthe interior of the body of the pipe section. In an alternativeembodiment, the conductive tube may be insulated from the pipe in orderto prevent possible galvanic corrosion. At present, the preferredmaterial with which to insulate the conductive tube 71 is PEEK®.

As shown in FIGS. 2 and 3, the coaxial cable 51 runs inside theconductive tube 71. As shown in FIGS. 6 and 10, the coaxial cable 51 hasa conductive core 79 surrounded by a dielectric sheath 81. In oneembodiment of the invention, the coaxial cable 51 also has a conductivesheath surrounding the dielectric sheath 81 and in electrical contactwith the conductive tube 51. The dielectric sheath 81 preventselectrical contact between the coaxial core 79 and the conductive tube71. As shown in FIG. 10, in a preferred embodiment, an inner layer ofthe dielectric sheath 81 is removed from around the conductive core 79at each end, while leaving the outer layer of the dielectric sheath 81in place next to the conductive tube 71. This allows insertion of theconnector 53 around the conductive core 79 and within the dielectricsheath 81. In another embodiment, a portion of the conductive sheath isremoved at both ends to thereby provide clearance for the conductivesleeve 75.

At present, the coaxial cable preferably has a characteristic impedancein the range of about 30 to about 120 ohms, most preferably with acharacteristic impedance in the range of 50 to 75 ohms. Because theattenuation of coaxial cable decreases with increasing diameter, thelargest diameter compatible with installation in pipe chosen for aparticular application should be used. Most preferably the cable has adiameter of about 0.25″ or larger. Preferably the shield should provideclose to 100% coverage, and the core insulation should be made of afully-dense polymer having low dielectric loss, most preferably from thefamily of polytetrafluoroethylene (PTFE) resins, Dupont's Teflon® beingone example. A foamed polymer may also be used as the core insulation.

It is preferable to select the electrical properties of the conductor soas to match the impedance of the coils to which it is attached.Preferably, the ratio of the impedance of the electrical conductor tothe impedance of the first and second electrically conductive coils isbetween about 1:2 and 2:1. Most preferably, it is close to 1:1.

The preferred data transmission system provides a relatively broadbandwidth. While not wishing to be bound by any particular theory, it iscurrently believed that this is accomplished by the low number of turnsof the conductor and the low reluctance of the magnetic path, thusproducing a surprisingly low mutual inductance for such a large diametercoil. For a two-turn coil with a 4.75-inch diameter, the mutualinductance of the assembled toroid is about 1 micro Henry. With a 50 ohmresistive load, peak signal transmission is at about 4 MHz, and at powertransmission extends from about 1 MHz to about 12 MHz. The inductivereactance is about 65 ohms, and the attenuation is only about 0.35 dBper joint, equivalent to power transmission of about 92 percent. In somerespect, the communication element is thought to perform as atransmission-line transformer, wherein the coupling between the adjacentcoils comprises distributed elements of both capacitance and inductance.Thus, the term “inductive coil” is intended to include both coils thattransfer signals via induction as well as those coils that act as atransmission-line transformer. As adjacent segments are assembled, aserial filter is created, which has the effect of reducing thebandwidth. If each individual transformer had a narrow bandwidth, theband-pass of the filter would change as additional segments are added,which would require that each individual element be separately tunedaccording to its position in the system. Nevertheless, a surprisingfeature of the invention is that identical segments can be assembled inany arbitrary number of joints while still enabling efficient signalcoupling. The 30-joint test described below gave a total attenuation of37.5 dB (0.018% power transmission), of which 70% was in the coaxialcable itself, which was chosen to have a shield diameter of 0.047inches. Maximum power transmission was at 4.2 MHz and the bandwidth, athalf power, of 2 MHz. Thus a six volt, 90 milliwatt signal resulted in adetected signal, after 30 joints, of 80 mV.

As shown in FIGS. 6, 9, and 10, in both the pin connector channel 31 andthe box connector channel 33 is a connector 53. The connector 53 permitsthe coaxial cable 51 to transmit an electrical signal to thecommunication element 91. The connector 53 has a conductive sleeve 75which fits around the conductive core 79. The connector 53 has aninsulative coating 77 to prevent electrical contact between theconductive sleeve 75 and the conductive tube 71. Preferably, theinsulative coating is TEFLON®. During assembly, the connector 53 ispushed over the conductive core 79, making electrical contact with it.Preferably the connector 53 makes spring contact with the conductivecore 79.

In a preferred embodiment, connector 53 fits around a wire 41, which isin electrical contact with the communication element 91. Most preferablythe wire 41 is one end of the conductive coil 87. The wire 41 ispreferably made of copper or silver-plated, copper-clad steel. The wire41 has an insulative coating 59, which is made of varnish, enamel, or apolymer. Most preferably, the insulative coating 59 is a tough, flexiblepolymer such as high density polyethylene or polymerizedtetrafluoroethane (PTFE). Preferably, the insulative coating 59 of thewire 41 is removed on the end of the wire 41 closest to the connector53, in order to facilitate electrical contact between the conductivesleeve 75 and the wire 41. In a more preferred embodiment, the connector53 is crimped around the wire 41 in order to ensure good electricalcontact between the conductive sleeve 75 and the wire 41.

In one embodiment of the invention, as shown in FIG. 9, a centeringinsulator 73 is used to help position the connector 53. The centeringinsulator 73 is funnel-shaped at each end and is made of a dielectricmaterial, preferably PTFE, most preferably PEEK®. The centeringinsulator 73 is hollow in the center, allowing it to slide around theconnector 53 and guide the connector 53 towards the core 79.

In a preferred embodiment of the invention, a water-tight seal 55, asshown in FIG. 6, is present in both the pin end connector channel 31 andthe box end connector channel 33 to protect the connections from thehigh temperature and high pressure downhole conditions. As shown in FIG.8, in a preferred embodiment, a spacer 65 is placed between theretaining plug 63 and the water-tight seal. Most preferably, the spacer65 is made of fiberglass. In the most preferred embodiment, the seal 55is located proximate to the retaining plug, as shown in FIG. 8, andforms a seal between the inner surface of the conductive tube 71 and theouter surface of the wire 41. In one embodiment, the seal comprises atleast one O-ring 67 and at least one backup 69. Most preferably, thereare at least three O-rings 67 and three backups 69. The O-rings 67 arepreferably made of rubber, more preferably fluoroelastomer, mostpreferably a fluoroelastomer marketed under the trademark AFLAS® VITON®.The backups 69 are preferably made of PEEK® and have a v-shapedindentation around one end. As an O-ring 67 is compressed, it moves intothe indentation in the backup 69 and causes the outer diameter of thebackup 69 to press against the conductive tube 71 and the inner diameterto press against the wire 41, thus helping to maintain the water-tightseal.

In an alternative embodiment, a water-tight seal is present between theconnector 53 and the inner surface of the conductive tube 71. In thisembodiment, the seal is provided by at least one circumferential grooveon the outside of the connector and at least one gasket fitting therein.Alternate embodiments may protect the connection with a water tight sealin other locations, such as between the coaxial core 79 and theconductive tube 71, between the connector 53 and the conductive tube 71,and between the wire 41 and the connecting channels 31 and 33.

Many types of data sources are important to management of a drillingoperation. These include parameters such as hole temperature andpressure, salinity and pH of the drilling mud, magnetic declination andhorizontal declination of the bottom-hole assembly, seismic look-aheadinformation about the surrounding formation, electrical resistivity ofthe formation, pore pressure of the formation, gamma raycharacterization of the formation, and so forth. The high data rateprovided by the present invention provides the opportunity for betteruse of this type of data and for the development of gathering and use ofother types of data not presently available.

Preferably, the system will transmit data at a rate of at least 100bits/second, more preferably, at least 20,000 bits/second, and mostpreferably, at least about 2,000,000 bits/second.

An advantage of the present invention is that it requires relatively lowpower and has a relatively high preservation of signal. Thus, the systempreferably transmits data through at least 30 components powered only bythe varying current supplied to one of the first conductive coils in oneof the components. More preferably, the system transmits data through atleast 50 components powered only by the varying current supplied to oneof the first conductive coils in one of the components.

Preferably, the varying current supplied to the first conductive coil inthe one component is driving a varying potential having a peak to peakvalue of between about 10 mV and about 20 V. Preferably, the power lossbetween two connected components is less than about 5 percent.

It is anticipated that the transmission line of the invention willtypically transmit the information signal a distance of 1,000 to 2,000feet before the signal is attenuated to the point where it will requireamplification. This distance can be increased by sending a strongersignal, with attendant increased power consumption. However, many wellsare drilled to depths of up to 20,000 to 30,000 feet, which wouldnecessitate use of repeaters to refurbish the signal. Preferably, theamplifying units are provided in no more than 10 percent of thecomponents in the string of downhole components, more preferably, nomore than 3 percent.

Such repeaters can be simple “dumb” repeaters that only increase theamplitude of the signal without any other modification. A simpleamplifier, however, will also amplify any noise in the signal. Althoughthe down-hole environment is thought to be relatively free of electricalnoise in the RF frequency range preferred by the invention, a digitalrepeater will provide a fresh signal without amplifying backgroundnoise. Most preferably, a “smart” repeater that detects any errors inthe data stream and restores the signal, error free, while eliminatingbaseline noise, is preferred. Any of a number of known digital errorcorrection schemes can be employed in a down-hole network incorporatinga “smart” repeater.

Most preferably, the repeater not only serves to regenerate the datastream, but also serves as a data source itself. Prior to the presentinvention, information was available during drilling only from thebottom-hole assembly, as mud pulse data rates did not allow anyintermediate nodes. With the present invention, information is availablefrom any node along the drill string, thereby enabling distributedaccess to information from top to bottom. For instance, instead ofrelying on a single bottom hole pressure measurement, a pressure profilecan now be generated along the entire drill string. This could be vitalin underbalanced drilling, where to speed up drilling the pressureprovided by the mud is less than that of the pore pressure in thesurrounding formation. Any sudden pressure pulse or “kick” could be muchmore rapidly anticipated. Other types of data sources for downholeapplications are inclinometers, thermocouples, gamma ray detectors,acoustic wave detectors, neutron sensors, pressure transducers,potentiometers, strain gages, seismic sources, and seismic receivers.

It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, that areintended to define the spirit and scope of this invention.

1. A system for transmitting data through a string of downholecomponents, each component having a first and second end, with the firstend of one component connected to the second end of an adjacentcomponent; the system comprising: a first communication element locatedat the first end of each component; a second communication elementlocated at the second end of each component, wherein each communicationelement comprises a first contact and a second contact; a coaxial cablerunning between the first and second communication elements, the coaxialcable comprising a conductive tube and a conductive core within theconductive tube; and a first and second connector for connecting thefirst and second communication elements respectively to the coaxialcable, each connector comprising a conductive sleeve, lyingconcentrically within the conductive tube, that fits around and makeselectrical contact with the conductive core, the conductive sleeve beingelectrically isolated from the conductive tube; wherein the conductivesleeve of each connector is in electrical contact with the first contactof the respective first or second communication element, and wherein theconductive tube is in electrical contact with the second contact of boththe first and second communication elements.
 2. The system of claim 1wherein the conductive tube runs the length of the coaxial cable.
 3. Thesystem of claim 2 wherein the conductive tube is made from a strongmetal.
 4. The system of claim 3 wherein the metal is steel.
 5. Thesystem of claim 4 wherein the conductive tube is held in tensionrelative to the down-hole component.
 6. The system of claim 1 whereinthe wherein the first and second communication elements are inductivecoils.
 7. The system of claim 6 wherein inductive coils are formed by asingle loop of wire.
 8. The system of claim 6 wherein the inductivecoils comprise at least one loop of wire set in a circular trough of amagnetically conducting, electrically insulating material, wherein thewire comprises a first end and a second end, which first end is thefirst contact and which second end is the second contact.
 9. The systemof claim 8 wherein the trough is formed of segments of a magneticallyconducting, electrically insulating material.
 10. The system of claim 9wherein the trough comprises ferrite.
 11. The system of claim 9 whereinthe segments are retained within a groove formed in a metal ring. 12.The system of claim 11 wherein the segments and the wire areencapsulated by an insulating material.
 13. The system of claim 12wherein the insulating material is selected from the group consisting ofpolyurethane, polytetrafluoroethylene, perfluoroalkoxy, or epoxy. 14.The system of claim 11 wherein the metal ring comprises a hole throughwhich at least the first end of the wire passes.
 15. The system of claim14 wherein the second end of the wire, the metal ring and the conductivetube are in electrical contact with each other, and the first end of thewire is in electrical contact with the conductive sleeve.
 16. The systemof claim 15 further comprising a retention bridge through which thefirst end of the wire passes and which insulates the first end of thewire from the metal ring.
 17. The system of claim 1 wherein thecomponents are sections of drill pipe, each having a central borepassing therethrough, and wherein the first and second communicationelements are located within a first and second recess respectively atthe first and second end of the drill pipe, and further comprising afirst passage passing between the first recess and the central bore anda second passage passing between the second recess and the central boreand wherein the first and second connectors are located in the first andsecond passages respectively.
 18. The system of claim 17 wherein eachsection of drill pipe has a portion with an increased wall thickness atboth the first and second ends with a resultant smaller diameter of thecentral bore the first and second ends, and wherein the first and secondpassages run through the portions with an increased wall thickness andgenerally parallel to the longitudinal axis of the drill pipe.
 19. Thesystem of claim 18 wherein the first and second passages each have alarger diameter portion and a smaller diameter portion, the largerdiameter portion being closer to the first and second recessesrespectively and the smaller diameter portion being closer to the end ofthe passage which exits into the central bore, and wherein the first andsecond connectors are located within the larger diameter portions of thefirst and second passages respectively.
 20. The system of claim 18wherein the conductive tube runs at least the length of the coaxialcable, and wherein the conductive tube has an outside diameter alongmost of its length that fits within the smaller diameter portions of thefirst and second passages and an increased outside diameter at both endswhich fits within the larger diameter portions of the first and secondpassages, but which increased outside diameter does not fit within thesmaller diameter portions, thereby retaining the coaxial cable in placewithin the section of drill pipe.
 21. The system of claim 1 wherein eachconnector further comprises a guide for the conductive sleeve, whichguide is made from a dielectric material and which includes a guidepassage through which the conductive sleeve passes, which guide passageis funnel-shaped at one or both ends.
 22. The system of claim 1 furthercomprising a water-tight seal between the outer surface of theconductive sleeve and the inner surface of the conductive tube.
 23. Thesystem of claim 22 wherein the seal is adapted to withstand pressures ofat least 25,000 pounds per square inch and temperatures of at least 200degrees Celsius.
 24. The system of claim 22 wherein the seal is providedby at least one O-ring.
 25. The system of claim 1 wherein the first andsecond communication elements each comprise an inductive coil having atleast one loop of wire, wherein in each communication element, the firstcontact comprises a first end of the wire and the second contactcomprises a second end of the wire, and wherein the first and secondconnectors each further comprise a water-tight seal between the secondend of the wire and the inside of the conductive tube.
 26. The system ofclaim 25 wherein the water-tight seal comprises at least one gasketthrough which the first end of the wire passes and which forms a sealwith the inner surface of the conductive tube.
 27. The system of claim26 wherein the seal is adapted to withstand pressures of at least 20,000pounds per square inch and temperatures of at least 200 degrees Celsius.28. The system of claim 26 wherein the gasket is made from a materialselected from the group consisting of pyrophylite, rubber fiberglass,and fluoroelastomer.
 29. The system of claim 28 wherein the gasketcomprises an O-ring.
 30. The system of claim 29 where each O-ring isadjacent to a locking ring, which locking ring tends to hold the O-ringin place under downhole conditions.
 31. The system of claim 25 whereinthe first and second connectors further comprise a second water-tightseal between the outside surface of the conductive sleeve and the insidesurface of the conductive tube.
 32. The system of claim 1 wherein thecoaxial cable further comprises a dielectric material between theconductive tube and the conductive core and running most of the lengthof the cable, but absent at both ends of the cable to thereby facilitateconnection to the first and second connectors.
 33. The system of claim32 wherein the coaxial cable further comprises a conductive sheathoutside of the dielectric material and inside the conductive tube, andwherein the conductive sheath runs most of the length of the cable butis absent at both ends of the cable to thereby facilitate connection tothe first and second connectors.
 34. A system for transmitting datathrough a string of downhole components, each component having a firstand second end, with the first end of one component connected to thesecond end of an adjacent component; the system comprising: a firstinductive coil located at the first end of each component; a secondinductive coil located at the second end of each component, wherein eachinductive coil comprises a wire with a first end and a second end; acoaxial cable running between the first and second communicationelements, the coaxial cable comprising a conductive tube and aconductive core within the conductive tube; and a first and secondconnector for connecting the first and second inductive coilsrespectively to the coaxial cable, each connector comprising aconductive sleeve, lying concentrically within the conductive tube, thatfits around and makes electrical contact with the conductive core, theconductive sleeve being electrically isolated from the conductive tube;wherein the conductive sleeve of each connector is in electrical contactwith the first end of the wire of the respective first or secondinductive coil, and wherein the conductive tube is in electrical contactwith the second end of the wire of both the first and second inductivecoils.
 35. The system of claim 34 wherein the inductive coils compriseat least one loop of wire set in a circular trough formed of segments ofa magnetically conducting, electrically insulating material.
 36. Thesystem of claim 34 wherein the trough comprises ferrite.
 37. The systemof claim 36 wherein the segments are retained within a groove formed ina metal ring.
 38. The system of claim 37 wherein the metal ringcomprises a hole through which at least the first end of the wirepasses.
 39. The system of claim 34 wherein the components are sectionsof drill pipe, each having a central bore passing therethrough, andwherein the first and second inductive coils are located with a firstand second recess respectively at the first and second end of the drillpipe, and further comprising a first passage passing between the firstrecess and the central bore and a second passage passing between thesecond recess and the central bore and wherein the first and secondconnectors are located in the first and second passages respectively.40. The system of claim 39 further comprising a water-tight seal betweenthe second end of the wire and the inside of the conductive tube.
 41. Ina system for transmitting data through a string of downhole components,the system comprising a plurality of downhole components, each with apin end and a box end, the pin end of one downhole component beingadapted to be connected to the box end of an other downhole component,each pin end comprising external threads and an internal pin face distalto the external threads, said internal pin face being generallytransverse to the longitudinal axis of the downhole component, and eachbox end comprising an internal shoulder face with internal threadsdistal to the internal shoulder face, said internal shoulder face beinggenerally transverse to the longitudinal axis of the down-holecomponent, and wherein the internal pin face and the internal shoulderface are aligned with and proximate each other when the pin end of theone component is threaded into a box end of the other component; a firstcommunication element located within a first recess formed in eachinternal pin face; a second communication element located within asecond recess formed in each internal shoulder face; and a conductor incommunication with and running between each first and secondcommunication element in each component; the improvement comprising: acoaxial cable as the conductor between the first and secondcommunication elements, the coaxial cable comprising a conductive tubeand a conductive core within the conductive tube; and a first and secondconnector for connecting the first and second communication elementsrespectively to the coaxial cable, each connector comprising aconductive sleeve, lying concentrically within the conductive tube, thatfits around and makes electrical contact with the conductive core, theconductive sleeve being electrically isolated from the conductive tube;wherein the first and second communication elements each comprises afirst contact and a second contact, wherein the conductive sleeve ofeach connector is in electrical contact with the first contact of therespective first or second communication element, and wherein theconductive tube is in electrical contact with the second contact of boththe first and second communication elements.
 42. The system of claim 41wherein the wherein the first and second communication elements areinductive coils.
 43. The system of claim 42 wherein the inductive coilscomprise at least one loop of wire set in a circular trough of amagnetically conducting, electrically insulating material, wherein thewire comprises a first end and a second end, which first end is thefirst contact and which second end is the second contact.
 44. The systemof claim 41 wherein each section of drill pipe has a portion with anincreased wall thickness at both the first and second ends with aresultant smaller diameter of the central bore the first and secondends, and wherein the first and second passages run through the portionswith an increased wall thickness and generally parallel to thelongitudinal axis of the drill pipe.
 45. The system of claim 44 whereinthe first and second passages each have a larger diameter portion and asmaller diameter portion, the larger diameter portion being closer tothe first and second recesses respectively and the smaller diameterportion being closer to the end of the passage which exits into thecentral bore, and wherein the first and second connectors are locatedwithin the larger diameter portions of the first and second passagesrespectively.
 46. The system of claim 45 wherein the conductive tuberuns at least the length of the coaxial cable, and wherein theconductive tube has an outside diameter along most of its length thatfits within the smaller diameter portions of the first and secondpassages and an increased outside diameter at both ends which fitswithin the larger diameter portions of the first and second passages,but which increased outside diameter does not fit within the smallerdiameter portions, thereby retaining the coaxial cable in place withinthe section of drill pipe.
 47. The system of claim 46 further comprisinga first and second expansion plug, each of which includes a centralpassage and each of which is press-fit within the conductive tube so asto maintain the increased outside diameter of the conductive tube withinthe larger diameter portions of the first and second passagesrespectively.
 48. A method of electrically connecting communicationelements at opposite ends of a downhole component through a coaxialconductor, the method comprising: providing a coaxial cable as theconductor between the first and second communication elements, thecoaxial cable comprising a conductive tube, a conductive core within theconductive tube and a dielectric material between the conductive tubeand the conductive core; providing a first and second connector forconnecting the respective first and second communication elements to thecoaxial cable, wherein the first and second connectors each comprises aconductive sleeve that fits around and makes electrical contact with theconductive core, the conductive sleeve being electrically isolated fromthe conductive tube; removing a portion of the dielectric material atboth ends of the coaxial cable to thereby provide clearance for theconductive sleeve; and sliding the first and second connectors over bothends of the coaxial cable.
 49. The method of claim 48 wherein thecoaxial cable further comprises a conductive sheath in electricalcontact with the conductive tube, and wherein a portion of theconductive sheath is removed at both ends to thereby provide clearancefor the conductive sleeve.
 50. The method of claim 48 wherein theconductive tube runs at least the length of the coaxial cable.
 51. Themethod of claim 48 wherein the components are sections of drill pipe,each having a central bore passing therethrough, and wherein the firstand second communication elements are located within a first and secondrecess respectively at each end of the drill pipe, and furthercomprising a first passage passing between the first recess with thecentral bore and a second passage passing between the second recess withthe central bore and wherein the first and second connectors are locatedin the first and second passages respectively.
 52. The method of claim51 wherein each section of drill pipe has a portion with an increasedwall thickness at both ends with a resultant smaller diameter of thecentral bore at both ends, and wherein the first and second passages runthrough the portions with an increased wall thickness and generallyparallel to the longitudinal axis of the drill pipe.
 53. The method ofclaim 52 wherein the first and second passages each have a largerdiameter portion and a smaller diameter portion, the larger diameterportion being closer to the first and second recesses respectively andthe smaller diameter portion being closer to the end of the passagewhich exits into the central bore, and wherein the first and secondconnectors are inserted within the larger diameter portions of the firstand second passages respectively.
 54. The method of claim 53 wherein theconductive tube runs at least the length of the coaxial cable, andwherein the conductive tube has an outside diameter along most of itslength that fits within the smaller diameter portions of the first andsecond passages and wherein the outside diameter of the conductive tubeis expanded at both ends after insertion into the larger diameterportions of the first and second passages, to thereby retain the coaxialcable in place within the section of drill pipe.
 55. The method of claim54 where the conductive tube is held in tension relative to the drillpipe.
 56. The method of claim 54 wherein the outside diameter of theconductive tube is expanded by inserting an expansion plug into eachend.
 57. The method of claim 56 wherein each expansion plug includes apassage through which a wire in electrical contact with the conductivecore passes.
 58. The method of claim 48 further comprising a guide forthe conductive sleeve, which guide is made from a dielectric materialand which includes a guide passage through which the conductive sleevepasses, which guide passage is funnel-shaped at one or both ends. 59.The method of claim 48 further comprising a water-tight seal between theouter surface of the conductive sleeve and the inner surface of theconductive tube.
 60. The method of claim 48 wherein the first and secondcommunication elements each comprise an inductive coil having at leastone loop of wire, wherein in each communication element, a first end ofthe wire is in electrical contact with the conductive tube and a secondend of the wire is in electrical contact with the conductive sleeve. 61.The method of claim 60 wherein the first and second connectors eachfurther comprise a water-tight seal between the second end of the wireand the inside of the conductive tube.